/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        ListNode* slow = head;
        ListNode* fast = head;
        ListNode* prev = nullptr;
        while (--n)
            fast = fast->next;
        
        while (fast->next)
        {
            prev = slow;
            slow = slow->next;
            fast = fast->next;
        }
        if (prev == nullptr)
        {
            ListNode* tmp = head;
            head = head->next;
            delete tmp;
        }
        else
        {
            prev->next = slow->next;
            delete slow;
        }
        return head;
    }
};